Download - Improvement of the PF ring vacuum system
Improvement of the PF ring vacuum system
Y. Hori*
High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan
Received 4 August 1999; accepted 3 February 2000
Abstract
For an upgrade of the PF ring, about half of the vacuum ducts were replaced by newly made or improved ones, which were
designed and manufactured to achieve the required beam lifetime. The concept of pumping was maintained. Every vacuum
duct was pre-baked and installed with care so as not to introduce contamination caused by exposure to the atmosphere. The
ring vacuum was successfully conditioned without an in situ bake-out. The operating pressure was speedily reduced by beam
cleaning as the operation time increased. A low operating pressure and resulting long beam lifetime was achieved within a
short operation period. # 2001 Elsevier Science B.V. All rights reserved.
Keywords: Synchrotron radiation; Light source; Vacuum duct; Beam lifetime
1. Outline of the reconstruction
The photon factory storage ring (PF ring) has been
operated as a dedicated synchrotron light source. As
one of recent activities, a new con®guration of the ring
was proposed and the ring was reconstructed [1,2].
The substance of the upgrading was to achieve low
beam emittance by reinforcing the focusing magnets
in the so-called `̀ normal-cell'' sections. In this project,
about half of the beam ducts of the ring had to be
replaced by new or improved ones to be compatible
with the new magnets. The ring was shut down from
December 1996 to September 1997 for the reconstruc-
tion.
Since neither the beam energy nor the bending
radius was changed before or after the reconstruction,
the outgassing rate per stored current per length by
photon stimulated desorption (PSD) must be almost
the same as before. Besides, the same ultimate pres-
sure as before is also enough for the new ring. Thus,
the pump location and pumping speed were main-
tained in order to guarantee a long beam lifetime. The
required pressure was below 4� 10ÿ8 Pa at 400 mA
of the stored beam. One of the key points was how
install bellows, beam position monitors (BPMs) and
pumping ports in a narrow space restricted by the
reinforcement of magnets.
2. Modi®cation of the vacuum duct
The main part of the replacement was carried out in
the normal-cell sections. There are two normal-cell
sections around the ring, and each section consists of
eight unit cells. Sixteen normal-cell ducts were neces-
sary for these sections. Since the bore radii of new
magnets were reduced compared to before, the cross-
sectional form of the beam duct was changed. The
design of the normal-cell duct was ®xed after the
fabrication of a prototype one. A major part of the
duct was made of aluminum alloy, as before. A typical
Applied Surface Science 169±170 (2001) 728±731
* Tel.: �81-298-64-5672; fax: �81-298-64-2801.
E-mail address: [email protected] (Y. Hori).
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 9 - 4 3 3 2 ( 0 0 ) 0 0 7 7 6 - 5
view of the duct is shown in Fig. 1. A bending part of
the old duct with an installed distributed ion pump
(DIP) was reused for the new duct because of a
budgetary restriction. This, however, resulted in dif®-
culties in the design, process and schedule. The old
ducts were removed from the normal-cell section, and
their bending parts were then improved and welded
with other prepared new parts during the shutdown. In
order to reduce any broadband impedance, the inner
walls of the ducts were connected as smoothly as
possible. Also, RF contacts were mounted in every
bellows and ¯ange mounted in the new ducts. Non-
circular bellows was adopted so that it could be
installed between the coils of the bending magnet.
Due to the limited space, no bellows could be mounted
between a crotch part and its downstream BPM, both
of which are ®xed points. In order to set these ®xed
points precisely, the normal-cell duct was assembled
within an accuracy of 1 mm. The pumping system was
not changed. The same three pumps (a titanium sub-
limation pump (TSP), an ion sputter pump (SIP) and a
DIP) were reused in every new duct. There is pumping
box between the bending part and its downstream
straight part. There are pumping slits on beam channel
in the box, of which total area is wider than 70 cm2 so
as to maintain an effective pumping speed. Only
extruded aluminum alloy ducts, from which the
straight ducts were made, were subjected to alkaline
etching before machining. The bending part was made
with machining, where ethyl alcohol was used as a
lubricant. No oil or grease was used. The machined
surface was subjected to no treatment by a chemical
attack. Every ®nished duct was ®lled with dry nitrogen
in the factory after it had passed a vacuum leak test,
and was brought to our facility.
The BPM was modi®ed according to the change in
the duct. Four pick-up electrodes were directly welded
on to the extruded straight duct, because the deviation
in the sensitivity caused by the nominal error of the
duct was simulated to be allowably small [3]. This
made the design and manufacturing of the duct simple
and easy. Every BPM was calibrated in order to
determine its electrical center. All of the measured
offsets were <500 mm, and typically 200 mm. This is in
good agreement with a simulation result obtained
from the boundary-element method, assuming a fab-
rication error. The measurement was reproducible
within 30 mm of dispersion. The BPM was mechani-
cally mounted on the quadrupole magnet within an
accuracy of 100 mm.
Some beam ducts were designed and fabricated
with special care. Four kicker ducts were fabricated
for the new kicker magnets. Their ceramic parts were
manufactured by sintering and planing. In order to
maintain a wide vertical aperture, the top and bottom
walls were made to be thin. Another beam duct with a
ceramic break was fabricated for a dc current trans-
former (DCCT) for measuring the stored beam cur-
rent. The existing DCCT had a problem of heat
generation caused by high-frequency noise leaked
from a ceramic break. A thin, wide ceramic plate
was used as a break for the new duct in order to reduce
the leakage. Its width and capacity are 0.5 mm and
3 nF, respectively. The same type coil as before is used
and its observed heat-up was effectively reduced
compared to the old DCCT [4].
3. Installation and evacuation
Every vacuum duct was pre-baked in our facility
and ®lled with dry nitrogen until installation to the
ring. The nitrogen was introduced through a micro-
®lter cooled by liquid nitrogen to eliminate water and
Fig. 1. Schematic drawing of a typical normal-cell duct.
Y. Hori / Applied Surface Science 169±170 (2001) 728±731 729
micro-dust. The normal-cell duct is directly laid on the
bending magnet. First priority of positioning is given
for the BPM. Because there is no bellows between the
®xed points of the SR port and its downstream BPM,
the SR port is set up with the error of manufacturing,
namely <1 mm. This leads to an angle error of less
than �0.7 mrad. A crotch absorber, TSP and BA
gauge were mounted in the duct after being pre-baked
as well as the beam duct. The SIP was kept in a
vacuum after being pre-baked to minimize pre-
adsorption. Dry nitrogen was always ¯owing through
the previously installed ducts during exposure those to
the atmosphere.
The average pressure of the ring decreased to
2� 10ÿ7 Pa just before beam injection on 1 October.
It was mainly determined at the normal-cell sections
where the local pressure was higher by roughly one
order than that at the other sections. No bake-out was
taken place after the installation, except at the section
where the RF cavities were installed, because it was
expected based on a previous experiment that the PSD
could be effectively reduced by SR irradiation, even
without in situ baking [5]. The yield per current was
very large in the beginning of ring operation. But it
became low enough to continue beam cleaning at
500 mA of the initial current after 1 A h of operation.
The TSP was refreshed whenever necessary to recover
the pumping speed.
4. Pressure and beam lifetime
The ring pressure is monitored every second by 48
BA gauges located around the ring. Two parameters
are conveniently used to evaluate the vacuum perfor-
mance independent of the stored current. One is the
product of the lifetime and the stored current (I � t) to
evaluate the lifetime. The other is the pressure normal-
ized to the stored current (p/I) to evaluate the pressure.
The pumping speed and its distribution was almost the
same; thus a required pressure (p/I) below
1� 10ÿ7 Pa/A must be achieved in the new ring.
The practical change in these two parameters is shown
in Fig. 2, where the operation time is represented by
the time-integrated stored current. The irradiating
photon dose is proportional to the operation time.
An operation time of 1 A h is equivalent to an aver-
aged photon dose of 3:89� 1022 photons/m. The
pressure speedily decreased as the operation time
was increased. The beam lifetime increased, being
inversely proportional to the pressure. The ring was
operated using the modulated optics until high-bril-
liant optics could be put into practice in May 1998.
The pressure once deteriorated around the time of
operation exchange, but gradually recovered again
with operation. A beam lifetime of 500 A min or more
was achieved in June, 1998.
The dominant process of beam loss in the ring was
collisions with residual gas molecules and the
Touschek effect in the low-pressure range. Fig. 3
shows the expected beam lifetime (I � t) as a function
of the pressure (p/I) of carbon mono-oxide for two
cases, each using modulated and high-brilliant optics.
The practical change in the lifetime is re-plotted in the
®gure. There is, of course, a difference between the
observed lifetime and the expected lifetime (calcu-
lated above), caused by the gas composition and gauge
location. The correction factor for them was estimated
to be 1.5. In any case, a simple inverse proportional
relation was observed. The beam lifetime became
longer as the pressure became lower. It was, however,
obvious that the lifetime was always shorter during
modulated operation than expected, just as the actual
acceptance was smaller than estimated.
Large outgassing was observed in the beginning
stage of high-current operation with low-emittance
Fig. 2. Beam lifetime and pressure as a function of the operation
time.
730 Y. Hori / Applied Surface Science 169±170 (2001) 728±731
optics. This resulted from a heat-up of the bumped
inner wall by shortened beam bunches. A number of
unshielded ¯ange gaps and bellows remained in the
unchanged sections. The outgassing was, however,
reduced during the tuning period of low-emittance
operation, and a low pressure was maintained during
user-mode operation. Though the pressure once dete-
riorated in the end of the modulated operation due to
consumption of TSPs, the lifetime maintained to some
extent, as shown in Fig. 2. It seemed that the Touschek
effect was also dominant during modulated operation
in the low-pressure range, as in the high-brilliant case.
On the other hand, the lifetime in the new operation
also agreed well with the expectation; thus, why the
observed behavior was contrary to the expectation
between the two operations is again incomprehensi-
ble. The reason is not yet clear.
References
[1] M. Katoh, et al., J. Synchrotron Radiation 5 (1998)
366.
[2] M. Katoh, et al., in: Proceedings of the 8th European Particle
Accelerator Conference, Stockholm, Institute of Physics
Publishing, 1998, p. 590.
[3] K. Haga, T. Honda, T. Kasuga, T. Obina, M. Tadano, in:
Proceedings of the 8th European Particle Accelerator Con-
ference, Stockholm, Institute of Physics Publishing, 1998,
p. 1517.
[4] T. Honda, Y. Hori, M. Tadano, in: Proceedings of the 8th
European Particle Accelerator Conference, Stockholm, Insti-
tute of Physics Publishing, 1998, p. 1526.
[5] Y. Hori, M. Kobayashi, Vacuum 47 (1996) 621.
Fig. 3. Expected beam lifetime as a function of pressure compared
with measured ones.
Y. Hori / Applied Surface Science 169±170 (2001) 728±731 731